In financial self-service apparatus, in order to check valuable documents (for example, a banknote, a check, a draft, etc.) processed in batches one by one, a whole stack of the sheet-like valuable documents are required to be separated one by one. Then a single sheet-like valuable document, which is separated, is subjected to identification processes such as an image recognition process, a thickness detection process and a magnetic information detection process, so as to ensure authenticity of the sheet-like valuable document. A process for detecting the thickness of the banknote is one essential identification process. At present, the sheet-like valuable document is generally made from special materials into a sheet-like medium having certain thickness characteristics. An adhesive tape adhered to the banknote and the deflection of the banknote may be identified by means of the thickness characteristics of the sheet-like valuable document, thereby eliminating unfit banknotes. Therefore, in the financial service apparatus, the reliable identification of the thickness of a valuable document is of great significance for checking the authenticity of the valuable document.
The structure principle of a conventional thickness sensor is shown in FIG. 1. One magnet 01 is mounted opposite to each linear Hall sensor 05. Entry of a sheet-like valuable document 02 into a conveying passage, formed between a reference shaft 03 and a floating roller 04, may cause a floating roller 04 to move upward, which causes the magnet 01 also to move upward. Thus, a magnetic field sensed by a Hall sensor 05 is intensified. The Hall sensor 05 is arranged on a printed circuit board (PCB) 06 and electrically connected to a control and calculation module. The displacement of the magnet 01 is calculated by detecting the change of the magnetic field, so as to detect the thickness of the sheet-like valuable document 02. Since the change of the magnetic field is non-linear, that is, the changes of the magnetic field are different when the magnet moves the same distance from a proximal end and a distal end of the magnet respectively, changes of the output of the linear Hall sensor are also different. Thus, what is detected by the sensor is not a linear relationship. When the thickness of the sheet-like valuable document to be detected is calculated based on the actual change of the detected magnetic field, however, a linear calculation method maybe adopted for convenience of calculation. Therefore, there is a large error in a detection result obtained in such a method.